Method for nitriding titanium surfaces



United States Patent METHOD FOR NITRIDING TITANIUM SURFACES James L. Wyatt, Cleveland, Ohio, and Nicholas J. Grant,

Winchester, Mass. assignors to National Lead Company, New York, N. Y., a corporation of New Jersey No Drawing. Application October 27, 1953, Serial No. 388,687

6 Claims. (Cl. 14813.1)

The present invention relates in general to nitriding metallic surfaces and more especially to an improved process for nitriding the surfaces of refractory metals.

The term nitriding, as it is commonly used, denotes a process for forming a hard surface or case on metals, and in particular iron, steel and alloyed steel products, by heating the iron or steel product in an atmosphere of ammonia gas at temperatures of approximately 800 F. to 1200 F and at about atmospheric pressure or slightly above to assist in excluding air from the treating zone, whereby the ammonia is dissociated to form molecular nitrogen and hydrogen, the nitrogen diffusing into the iron or steel to produce a hard case.

The term case as used herein shall be understood to denote the zone of a specimen effected by inward diffusion of a solute species and is to be distinguished from the core of a specimen which is that zone unalfected by the phenomenon of diffusion. Moreover, since the instant case relates in particular to nitriding a refractory metal by the use of ammonia gas, the solute species referred to herein is a nitrogen species of undefined charactor but which has been postulated to include molecular nitrogen, atomic or nascent nitrogen; or any of several nitrogen-hydrogen radicals, i. e. (NH); or ionic forms of any of these.

Among the more recent developments in fields of metallurgy has been the production of titanium metal and its alloys. While titanium metal exhibits many properties that are attractive not only to specialty fields but also to the more common construction uses, the metal is characterized by several properties that have led to difficulties in fabricating and using the fabricated metal. Among these is the tendency of the metal and its alloys to seize and gall under conditions of use, wear, or when extruded, drawn and the like.

Some efforts have been made heretofore for nitriding titanium metal, i. e. for producing a hard adherent case on metallic titanium, one such process being that of subjecting the metallic titanium to the action of a molten alkali metal cyanide as disclosed in the Dean Patent, 2,453,896, November 16, 1948, for Treatment of Titanium. In the thesis of James L. Wyatt entitled In vestigations of Titanium Cyaniding, Carbonizing and Nitriding submitted in partial fulfillment of the requirement for the degree of Doctor of Science from the Massachusetts Institute of Technology, 195 3, are reported the results of an intense and extensive investigation of the whole field relating to nitriding as applied to refractory metals, and in particular titanium and zirconium. Among the investigations made is that relating to the subject matter of the aforesaid Dean patent. In brief, specimens of titanium were immersed in liquid sodium cyanide baths of varying compositions, under inert and 2,804,410 Patented Aug. 27, 1957 reactive atmospheres, and at temperatures up to 1600 F., and in every instance it was found that the corrosion rate was actually of such magnitude as to preclude the formation of a case and hence rule out the Dean process as a commercial means for surface hardening titanium metal.

The instant invention had its inception in the exhaustive research work carried out under the above entitled thesis and has as one of its objects that of providing a process for surface hardening refractory metals and their alloys whereby the metals will have improved corrosion resistance, low coeflicients of friction, improved creep rupturelife and satisfactory strength characteristics at inordinately high temperatures.

A further object of the present invention is to provide a surface treatment for refractory metals and their alloys whereby the metals will have highly improved wear characteristics.

A still further object of the invention is to provide an improved nitriding treatment for titanium and zirconium metals and their alloys for substantially eliminating the propensity of such metals for galling during wear, drawing, extruding and similar load bearing applications.

In general, the instant invention is based on the discovery that refractory metals, and in particular titanium metal and its alloys, such as for example the titanium alloys of aluminum and chromium, can be provided with a hard case by treatment with ammonia gas under carefully regulated conditions which are critical for the successful production of a hard case on a titanium metal surface.

In its broadest aspects, therefore, the invention relates to an improved process for producing a hard case on a refractory metal surface by confining the refractory metal in a heated zone and passing ammonia gas through the zone at a predetermined rate to provide an oxygen free atmosphere therein comprising partially dissociated ammonia, and a maximum of about 2%, by volume of the total gas introduced, undissociated ammonia, but preferably from about 0.05% to about 0.2% undissociated ammonia; the nitriding action of the dissociated ammonia, and in particular the nitrogen species in contact with the refractory metal surface being carried out at temperatures dependent on the rate of feed of the ammonia and for periods of time dependent on the temperature to form a high hardness case of maximum thickness on the refractory metal surface.

It was quickly realized at the outset that a process for nitriding titanium cannot be patterned after the processes now used for nitriding steel. An initial series of tests indicated not only that ammonia purity and dryness are extremely important to the successful formation of a hard case on titanium metal but that the rate of flow of the ammonia gas through any particular heated zone is also an important factor in producing solid eases free of voids. Moreover, it has been discovered that this same factor, namely the requirement for passing the ammonia gas through a particular heated nitriding zone at a predetermined flow rate may be expressed more generally in terms of the amount of undecomposed or undissociated ammonia gas in any selected nitriding zone. Specifically, experimental results show that while the atmosphere of the nitriding zone should be oxygen free and composed primarily of dissociated ammonia gas including a nitrogen species, there must be, nevertheless, a maximum of about 2% undissociated ammonia gas in the nitriding zone, and preferably from about 0.05% to about 0.2%

by volume of the total gas introduced, to insure consistently uniform results.

It is postulated that the case on the titanium is created by a nitrogen solute occluded in the lattice structure of the metal, and, as pointed out above, experimental results have shown that the amount of nitrogen solute occluded in the lattice structure of the titanium metal surface, as evidenced by the weight gain in milligrams per square decimeter of a given specimen attains an optimum value at an ammonia flow rate which is dependent upon such factors as the size of the furnace chamber, the total surface area of metal exposed and the temperature, the ultimate and more general requirement, in any case, being that of insuring an efliuent gas composition in the nitriding zone of as high as 2% undissociated ammonia gas.

In addition to these factors, it was discovered that in nitriding titanium metal the nitriding time is critical for a case of any given thickness, i. e. the thickness of the case increases substantially linearly with the nitriding time up to a critical point after which the thickness of the case decreases rapidly with further treatment. This phenomenon appears to be peculiar of titanium metal. Similarly, titanium metal treated at a predetermined temperature for an optimum number of hours exhibits much higher hardness values at a predetermined depth below its surface than when treated at the same temperature for longer periods of time than the optimum. In general, hardness values of a titanium metal case produced under the critical conditions set forth above are in the range of from 1400-1700 Khn. (Knoop hardness number), and these case hardness values were substantially the same irrespective of the time and temperature employed, the major eifect of time and temperature being to produce higher hardnesses in the core zone of the material.

Concerning the nature of the case formed on titanium metal by nitriding with ammonia gas in the manner of the instant invention, it was observed that a thin layer or patina of titanium nitride (TiN) is formed on the surface of the case which imparts a characteristic gold color to the case. However, there was no evidence that the titanium nitride patina provides any hardening effect, and it was found that the nitride patina could be readily removed from the case with a rotary wire brush without changing the measured hardness of the case. Immediately beneath the patina of the case is a layer of structureless appearance having little or no grain delineation, the thickness of which varies from to a maximum of about 0.005 inch. This layer is identified as the high hardness case of the metal, the thickness of which is, as set forth above, a function of both treatment time and of treatment temperature, there being a definite treatment period for each temperature to yield a case of maximum thickness. It is postulated that this case consists of a solid solution of nitrogen or a nitrogen-hydrogen radical in alpha titanium and that the high hardness of the case is due to solid solution hardening.

With respect to optimum nitriding time for producing a high hardness case of maximum depth, it was found that a Temperature range of from 1400-1700 F., and a nitriding time of from about one to as high as 200 hours were capable of producing optimum cases of maximum hardness and maximum depth.

In the foregoing description it is evident that the instant invention embodies the discovery that the nitriding of titanium metal may be accomplished successfully to produce a case of high hardness and appreciable thickness by subjecting the titanium metal to ammonia gas in conjunction with the three major inter-related variables of time, temperature and rate of gas flow, the latter factor being expressed in terms of the amount of undissociated ammonia gas in the nitriding zone, there being a critical treatment for each temperature employed in order to produce a case of maximum thickness. To summarize, the table below itemizes the aforesaid inter-related variables and designates the specific ranges to be employed for producing a high hardness case of maximum depth on titanium metal.

Tabulation of NH: nitriding factors Percent Un- Time dissociated Case Temp. ("I (Hours) NH; By Vol- Thickness urnc Gas (In. 10- Introduced -200 0. 15-0. 2 4-5 60-90 (1. 15-0. 2 4-5 30-45 0. 15-0. 2 4-5 20-30 0. 15-0. 2 4-5 10-20 0. 15-0. 2 4-5 l-B 0. 15-0. 2 4-5 Less than 2 0. 15-0. 2 4-5 In all cases the titanium metal used was a hot rolled commercial purity grade metal.

In carrying out the process of the instant invention for ammonia nitriding titanium metal, suitable apparatus will comprise a furnace in which is mounted a nitriding chamber which provides the reaction or nitriding zone, and to which the ammonia gas may be fed at one end and exhausted at the opposite end of the chamber. The nitriding chamber is adapted to be sealed from the atmosphere to preclude admission of oxygen into the nitriding zone of the chamber; and is heated by external heating means, as for example by burners in the furnace surrounding the chamber or by electrical heating units, such as for example resistance coils or induction heating equipment. As pointed out earlier, one of the important prerequisites to the formation of a satisfactory case is the use of a substantially pure ammonia gas free from moisture, and hence it is necessary to interpose ammonia purifying equipment, including a drying tower between the ammonia source and the ammonia inlet of the nitriding chamber.

Prior to initiating treatment of the titanium metal, the latter is placed in the nitriding zone of the chamber whereupon the latter is flushed with nitrogen or ammonia to remove any air or other deleterious gas. The temperature of the nitriding chamber is then raised to within the temperature range specified above whereupon ammonia gas is fed to the nitriding chamber at a rate determined by the temperature used for nitriding the titanium metal, such as to insure an atmosphere of dissociated ammonia including up to about 2% and preferably from about 0.05% to about 0.2% undissociated ammonia in the nitriding chamber by volume of the gas introduced. The flow of ammonia gas through the nitriding chamber is maintained constantly for a period of time which is also a function of the temperature used for nitriding the titanium metal. At the end of this time which is critical for the production of a high hardness case at the nitriding temperature employed, the gas flow is cut off, the temperature lowered, and the nitrided titanium metal removed from the chamber.

The high hardness case formed on titanium metal by the process of this invention exhibits a sliding friction coefiicient among the lowest ever reported for metals and has been measured as low as 0.08.

In addition to completely eliminating galling and seizing and producing an exceptionally low sliding friction coefficient, the high hardness case formed on titanium metal by the nitriding process of this invention has been found to increase the elevated temperature creep rupture-life of the titanium metal and its alloys by as much as from two to forty fold, as a consequence of which the temperature range of applications of the nitrided titanium metal is increased. Moreover, extensive experiments have shown that the oxidation and corrosion rates of nitrided metal are much lower, being in the order of 15 of that for untreated titanium.

The tables below compare the above mentioned properties of titanium metal and its alloys before and after being treated by the nitriding process of this invention.

Coefficient of sliding friction Conditions Before After Nltriding Nltriding Mild Steel on Tl Metal 0. 51 0. 36 Mild Steel on Ti Met 0. 49 0. 32 Hard Steel on T1 MetaL. I). 26 Nltrided TI on N itrided Ti (Un1ubricated).... "A- 0. 137 N itrided 'Ii on N itrided Tl (Lubricated) 0. 080

l The titanium metal (Tl) used was a hot rolled metal of commercial purity.

Corrosion rates-Weight loss in grams The high hardness case formed on titanium metal in accordance with the process of this invention is effected by the use of ammonia gas, in the manner hereinabove described, and although one might expect a similarity of result by using nitrogen gas, experiments have indicated that only by treating the metal over long periods of time and at unusually high temperatures may any significant hardening be noted. Consequently, from both an economic and commercial viewpoint, case hardening of titanium metal by nitrogen gas is impracticable.

As pointed out earlier, the nitriding process of this invention is not only applicable to titanium metal and its alloys but may be used successfully for nitriding zirconium metal. In general, it has been found that zirconium metal nitrided by the process of this invention absorbs more of the hardening species than titanium for a given treatment time and that the depth of the case increases with increased nitriding time without there being any critical time such as characterizes the nitriding time of titanium metal. Hence, it is possible to achieve a thicker high hardness case on the surface of zirconium although the maximum hardness recorded for zirconium is about 950 Knoop which is appreciably lower than the high hardness values of the case formed on titanium metal.

From the foregoing description it is manifest that the present invention discloses a process for nitriding refractory metal surfaces, and in particular the surface of titanium metal and its alloys as well as zirconium metal to form a high hardness case which not only substantially eliminates galling and seizing but increases the corrosion resistance of the metal as well as maintaining the strength characteristics of the metal at increased temperatures, as a consequence of which the case hardened metal is provided with improved wear applications as well as improved qualities for fabrication.

While this invention has been described and illustrated by the examples shown, it is not intended to be strictly limited thereto, and other variations and modifications may be employed within the scope of the following claims.

We claim:

1. A process for nitriding a refractory metal surface which comprises: heating the surface of a refractory metal selected from the group consisting of titanium and zirconium to a temperature within the range of from about 1400 F. to about 1700 F. in an oxygen free atmosphere of partially dissociated ammonia gas including undissociated ammonia in an amount from 0.05 to 2% by volume of the gas introduced into said atmosphere; and continuing said heating for a period of time which bears an inverse relationship to the temperature of said heated atmosphere to form a high hardness case on said refractory metal.

2. A process for nitriding a titanium metal surface which comprises: heating the surface of titanium metal to a temperature within the range of from about 1400 F. to about 1700 F. in an oxygen free atmosphere of partially dissociated ammonia including from 0.05% to 0.2% undissociated ammonia by volume of the gas introduced into said atmosphere; and continuing said heating for a period of time which bears an inverse relationship to the temperature of said heated atmosphere to form a high hardness case of said titanium metal.

3. A process for nitriding the surface of a refractory metal selected from the group consisting of titanium and zirconium which comprises: maintaining an oxygen free atmosphere of partially dissociated ammonia in a reaction zone by passing ammonia gas through said reaction zone at a rate such as to insure the presence therein of undissociated ammonia in an amount from 0.05% to 0.2% by volume of the gas introduced into said zone; confining said refractory metal surface in said atmosphere of partially dissociated ammonia; subjecting said refractory metal surface to the action of said partially dissociated ammonia by heating said refractory metal surface to a temperature within the range of from about 1400 F. to about 1700 R; and continuing said heating for a period of time which bears an inverse relationship to the temperature used in heating said refractory metal surface to form a high hardness case of maximum thickness on said refractory metal.

for nitriding a refractory metal surface maintaining an oxygen free atmosphere of partially d socinted ammonia in a reaction zone by passing ammonia gas through said reaction zone at a rate such as to insure the presence therein of undissociatcd ammonia in an amount from 0.05% to 0.2% by volume of the gas introduced into said zone; confining the surface of a refractory metal selected from the group consisting of titanium and zirconium in said atmosphere of partially dissociated ammonia; subjecting the refractory metal surface to the action of said partially dissociatcd ammonia by heating said refractory metal surface to a temperature within the range of from about 1400 F. to about 170W F.; and continuing said heating for a period of time within the range of from about 1 hour to about 200 hours which time bears an inverse relationship to the temperature used in heating said refractory metal surface to form a high hardness case of maximum thickness on said refractory metal.

5. A process for forming a high hardness, titanium nitride case on the surface of titanium metal which comprises; heating said titanium metal in an oxygen free nitriding zone having an atmosphere consisting of partially disscchu-zd ammonia gas including undissociated ammonia gas in an amount from 0.05 to 2% by volume of the ammonia gas introduced into said zone; and continuing said heating for a period of time which bears relationship to the temperature of said atmosphere about as follows:

Time (hours): Temp. F.) -200 1395 60-90 1445 30-45 1495 20-30 1545 10-20 1595 1-3 1645 Less than 2 1695 6. A process for forming a high hardness zirconium Time (hours): Temperature F.) nitride case on the surface of zirconium metal which 150-200 1395 comprises: heating said zirconium metal in an oxygen 0-90 1445 free nitriding zone having an atmosphere consisting of 30-45 1495 partially dissociated ammonia gas including undissociated 5 2030 1545 ammonia gas in an amount from 0.05% to 0.2% by 2 1595 volume of the ammonia gas introduced into said zone; l3 1645 and continuing said heating for a period of time which LESS than 2 1695 bears relationship to the temperature of said atmosphere about as f ll 10 References Cited in the file of this patent WAL Report No. 401/49-9 published by Watertown Arsenal Lab., Mass, pages l0-21. 

1. A PROCESS FOR NITRIDING A REFRACTORY METAL SURFACE WHICH COMPRISES: HEATING THE SURFACE OF A REFRACTORY METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM AND ZIRCONIUM TO A TEMPERATURE WITHIN THE RANGE OF FROM ABOUT 1400*F. TO ABOUT 1700*F. IN AN OXYGEN FREE ATMOSPHERE OR PARTIALLY DISSOCICATED AMMONIA GAS INCLUDING UNDISSOCIATED AMMONIA IN AN AMOUNT FROM 0.05 TO 2% BY VOLUME OF THE GAS INTRODUCTION INTO SAID ATMOSPHERE; AND CONTINUING SAID HEATING FOR A PERIOD OF TIME WHICH BEARS AN INVERSE RELATIONSHIP TO THE TEMPERATURE OF SAID HEATED ATMOSPHERE TO FORM A HIGH HARDNESS CASE ON SAID REFRACTORY METAL. 